Project Summary/Abstract An increasing body of evidence indicates that the state of activation, phenotype and functional response of the microvasculature to alloimmune injury creates an intragraft microenvironment that both initiates and sustains the progression of allograft rejection. The unique anatomic location of the graft endothelial cell (EC) ensures that it is a target of the alloimmune response, and characteristic phenotypes either augment or inhibit interactions with immune cells. We recently identified DepTOR as a cell intrinsic mTOR-binding partner that modulates mTOR, MAPK and STAT-induced signaling in microvascular EC. In addition, we found that it is potent to regulate activation responses, angiogenesis and EC-dependent mechanisms of inflammation in vitro. These observations allowed us to conclude that the removal of its negative/regulatory effects are a mechanistic component of the EC pro-inflammatory phenotype. Our findings also suggest that augmented levels of expression and/or function of DepTOR may inhibit intragraft EC activation and their immunogenicity to sustain immunoregulation in vivo. However, formal testing of all these possibilities requires models in which DepTOR expression may be regulated in vivo selectively within allografts post transplantation. In this R21 proposal, we plan to develop and use novel DepTOR transgenic mice as donors of cardiac allografts in well-established models in vivo. We will also use cultured EC from transgenic mice for mechanistic studies in vitro. In this manner, we will be able to evaluate whether DepTOR is functional within allografts to promote long-term graft survival, and whether it effects are dependent on its ability to regulate EC-dependent mechanisms of rejection. Our central hypothesis is that cell intrinsic expression of DepTOR within the graft is immunomodulatory and regulates EC activation responses, EC-dependent interactions with CD4+ T cells and acute and chronic rejection. We will test this hypothesis in two specific aims in which we will 1), determine the function of DepTOR within the graft, and its association with acute and chronic allograft rejection in vivo, and 2), determine the function of DepTOR in EC-dependent mechanisms of alloimmunity. Collectively, our proposed studies are novel and address clinically relevant questions and our approach provides for cohesiveness to translate in vitro findings into relevant transplant models in vivo. These exploratory studies will, for the first time, define a mechanism whereby the expression of a regulatory protein within the transplanted organ itself may be a determinant of the phenotype and outcome of the rejection process.